1. Field of Invention
This invention is in the field of signal processing to enhance the effective signal to noise ratio of the received signal by coherently summing the signals with different carrier frequencies and carrier phases but with a common modulation.
2. Related Art
An adaptive phased array, where the signals with a common carrier frequency and a common modulation, but with different carrier phases, are summed in phase, may be regarded as a relevant prior art. When the incident signals have different carrier frequencies, the in-phase summation concept by adjusting weights in the antenna element circuits, is no longer effective. Also, when signals are not summed in phase, no improvement of the signal-to-noise ratio of the received signal can result. The present invention comprising means and method to sum the signals with a common modulation, but having different carrier frequencies and carrier phases, provide an improvement in effective signal-to-noise ratio.
The communication between a moving platform, such as an airplane or a ship, and a ground station, becomes ineffective as the distance between the platform and ground station exceeds a certain limit that depends primarily on the transmitter power of the communication system. When an effective communication of information to the platform is necessary at all times, a number of ground stations, each transmitting the same information, are used on the basis that the platform will be close to at least one of the ground stations to receive the information. To avoid a destructive interference among the signals transmitted by many ground stations the platform tunes to the frequency of its closest ground station. The same number of ground stations can be used to communicate with many moving or fixed platforms, thus making the communication network more cost effective when there are more than one moving platform. When the location of the platform is very arbitrary over a very large area, the number of ground stations necessary for a reliable communication could be prohibitively large. Since the information communicated by each ground station is the same, the number of ground stations can be significantly reduced if one could sum the information coherently. But the information is contained in the modulation of the signal transmitted by each ground station. Thus a coherent summation of the common modulations of the otherwise incoherent transmitted signals from various ground stations as received at the platform, will greatly enhance communication effectiveness because of the equivalent added signal strength. One objective of the invention is to sum coherently the common modulation when the carrier frequencies and phases of the signals are not the same and are changing with time.
Another objective of the invention on combining multifrequency signals with a common modulation, is to provide a unique identification means for a given platform. Thus, when a modulated interrogating signal is communicated to a platform and the platform, in turn, acts as a transponder sending out a plurality of signals each having a carrier frequency which is obtained by a frequency offset from the carrier frequency of the interrogating signal, the total radiated transponder signal from the platform may be viewed as a set of multifrequency signals having different carrier frequencies and carrier phases but having a common modulation. If the set of offset frequencies are unique to a platform, then combining the modulations at these specific offset frequencies provides a means of recognition of that particular platform. Moreover, if the set of offset frequencies to be used at the platform is not known to the receiver of the multifrequency signals, no signal combining will be possible. Hence a system can be designed such that the identification of a platform can only be possible by an authorized interrogating statio to synchronize the carrier frequencies and their phases for a number of signals. Such a synchronization by adaptive means requiring no a priori information on the characteristics of the signal or its modulation is the key element of the present invention. No prior art on this specific aspect of the invention is known to this inventor.
Differences between this invention and any related prior arts will be apparent from the specifications, drawings, and claims that follow.